1. Field of the Invention
The present invention generally relates to transmission cables and, more particularly, is concerned with an apparatus for providing a rotary coupling between stationary and rotatable parts of transmission cables which permits their relative rotation without disrupting signal transmissions, whether electrical or optical, through the cables and between the stationary and rotatable parts thereof.
2. Description of the Prior Art
In many industries in general there is increasing use of computer technology to control the operation of machines and the movement of products. In many instances, such use of computer technology requires high quality transmission networks. Within the container shipping industry in particular there is increasing tendency towards tracing cargo by using computer technology. The achievement of computerized cargo traceability requires the use of high quality fiber optics in data transmission networks.
The data transmission cables, together with the power supply cables, of such networks have to be managed in some way when connected to moving machinery. Since the power supply cables are typically handled by the use of rotatable cable reels, it would then seem to readily follow that the transmission cables could be easily handled by placing them on the rotatable reels with the power cables. However, unlike the electricity being transmitted on power supply cables which can be "collected" by using slipring and brushgear assemblies, the bands of light in fiber optics have to be transmitted as a continuous unbroken beam. Effective transmission of continuous unbroken beams of light between stationary and rotating parts of transmission cables is thus more complicated than merely placing the transmission cables on the rotatable reels with the power cables.
Heretofore, the continuous transmission of light beams through fiber optics having relative rotating parts has been achieved using one of two methods. The first method involves the use of an optical rotary joint but this method is limited to a low number of passes through the cable. With respect to a single pass the optical joint is relatively straightforward, economical and reliable; but for two or more passes the joints become highly sophisticated in their optical engineering and are very expensive. However, the joints do have the advantage of being capable of rotating in either direction indefinitely. Several models of a device employing this first method are manufactured by Focal Technologies Inc. of Dartmouth, Nova Scotia, Canada.
The second method involves the use of some kind of transfer mechanism whereby a continuous length of fiber optic cable is wound and unwound from adjacent spools as a central shaft rotates. The winding and unwinding of the cable is such that it eliminates twisting of the cable. This type of device is limited to the relatively small number of rotations that can take place in one direction before it has to be reversed. Several models of a device employing this second method are manufactured by Stemmann-Technik GMBH of Schuttorf, Germany, and by Specimas Spa of Nova Milanese, Italy.
Consequently, a need exists for improvements which overcome the limitations of the aforementioned prior art devices without introducing any new limitations in their place.